1. Field of the Invention
The present invention relates to a refiner for refining material for use in the production of paper, paper-related products and other products. In particular, the present invention relates to a refiner disc which includes an exhaust channel extending through the body of the disc for venting high pressure steam and/or other vapor during the refining process and a method for relieving pressure buildup between two opposed refiner discs through the use of such an exhaust channel.
2. Discussion of the Related Art
A refiner is a machine that mechanically treats and breaks down wood chips and clumps of fibers into individual pulp fibers from which paper may be made. In a rotary disc refiner, pulp fibers are shredded between closely spaced discs. A refiner disc consists of a round steel or steel alloy plate which has a generally flat grinding surface comprised of a multiplicity of generally radial ribs forming generally radial grooves between them. Typically, a first disc is mounted on a rotor for rotation about an axis, and a second disc is disposed in a position opposing the first disc. Alternatively, the second disc may be mounted on an oppositely rotating rotor so that the two discs rotate about the same axis of rotation but in opposite directions. In either case, the relative motion of the ribs and grooves on the grinding surfaces of the two opposing discs rolls and tears, shears and grinds the bulk fiber material into finely ground pulp fibers.
In refining pulp, friction during the refining process produces a large amount of heat. Of course, pulp fibers have at least some amount of water content, and in some refining operations water or other chemical solutions may be added to the fiber mass during the refining process. Additionally, these fibers typically are transported to and from the refiner in a watery slurry. The temperatures produced during refining, however, are sufficiently high to vaporize the water and therefore a great amount of steam typically is generated during the process.
In analyzing the steam pressure versus the radius of the refining disc, peak steam pressure is typically located approximately at the midpoint of the disc. It is believed that steam pressure is highest in this area of the disc because the number of ribs and grooves is greater at the midsection. In looking at a typical refining disc, the radially interior portion of the disc is comprised of a relatively few widely spaced ribs and grooves for gross break-up of the bulk material. During the process, centrifugal force produced by the rotation of the disc causes the fibers to migrate from the center of the disc to the outer portion of the disc. At approximately the midsection, as well as the outer section of the disc, the number of ribs and grooves is increased, and the spacing between the grooves is much narrower in comparison to the center of the disc. The mid and outer sections of the disc are commonly referred to as the "refining zone" because it is in this area that the fibers are most finely ground. In any event, because the amount of friction is higher in the midsection of the disc, temperatures and steam pressures are also higher. It is also believed that steam pressure is highest at the midsection of the discs because that is the most confined area of the refining zone. The causes of peak steam pressure at the midsection of the refining discs is further explained and illustrated in, for example, B. J. Allison, et al., "Grey-Box Identification of a TMP Refiner," 1995 International Mechanical Pulping Conference, pages 119-24, and Per Engstrand, "The Impact of Chemical Addition on Refining Parameters," 1995 International Mechanical Pulping Conference, pages 281-86.
High steam pressure at the midsection of the refining discs causes a number of operational difficulties. First, high steam pressure generated during the refining process tends to force the discs apart thereby reducing the amount of force the discs actually applies to the fiber lessening their treatment. The force of the steam also causes the disc to oscillate to some degree as it rotates. Of course, larger motors and thrust bearings can be used to compensate for the energy wasted on simply forcing the plates together, but obviously at a higher cost. Second, high pressure steam in the midsection of the refining plate tends to blow some wood pulp out of the refining zone prematurely before it is adequately refined. In some cases the steam forms a back pressure which restricts the movement of wood pulp through the refining zone. Both situations can cause an inconsistency in the refined pulp fibers.
Others have of course sought to address the undesirable effects of high pressure steam generated during refining. For example, U.S. Pat. No. 5,373,995 issued to Ola M. Johannson on Dec. 20, 1994, discloses a refining disc having a number of dams in the grooves, and apertures in the ribs of the disc. The dams in the grooves block the radial movement of the pulp fibers, while the apertures in the ribs provide passages for venting the steam out of the refining zone. While the dams tend to reduce the amount of fibers being prematurely blown out the refining zone, the apertures in the ribs do not necessarily alleviate the very high pressure generated at the midsection of the refining plate. Another example is U.S. Pat. No. 4,676,440 issued to Markku Perkola on Jun. 30, 1997, which discloses a series of deep arc-shaped channels on the face of the disc for exhausting steam. However, because the large channels take up so much of the space on the face of the disc, the amount of surface area for actual refining is greatly reduced. Consequently the overall effectiveness and consistency of the refining is greatly reduced. A second problem with Perkola is that a significant amount of insufficiently refined fibers are drawn into the deep channels and discharged from the refining zone prematurely.
Consequently, a rotary disc refiner for refining wood pulp which includes an effective means for venting steam from a high pressure zone of the refining disc to a low pressure zone, and which will not adversely affect the consistency of the refined fibers is desired.